what would you like to say specifically?
A few ideas:
Palju õnne ja rõõmu teie pulmapäeval! (Much happiness and joy for your wedding-day)
Südamlikud õnnesoovid pulmapäevaks! (Congratulations from the heart on your wedding)
Pikki aastaid ja armastust! (Long years and love)

It might be too late by now, but let me know if I can translate a phrase for you.

No, it's not too late! A friend of mine is getting married in September and I have an idea of writing her best wishes in a card. Since I'm in love with foreign languages, I decided to write down some phrases in some languages which are not usual or widely popular, but to me those languages are way more interesting than English, French or German, for example If only I could learn Estonian in my country! It would be brilliant, but, unfortunately, impossible...
The phrases you've written are great, thank you very much! Or should I say - tänan väga?